Fiber glass boards are sometimes used as form boards to temporarily support poured lightweight concrete or gypsum. When set, the resulting slab functions as a roofing surface or an interstitial limited-access maintenance floor between main floors in a building such as a hospital. Fiber glass boards are well suited for such use. They are lightweight, fire resistant, easy to handle and can be cut to size to fit around obstacles such as pipes, cables and columns. In addition, their resin bonded fibers are strong, lying generally in a direction perpendicular to the applied load and being capable of transferring and distributing the load uniformly. Fiber glass boards also absorb a portion of the water in the concrete or gypsum slurry and thus aid in the setting of this medium. Such boards typically have a thickness of one to two inches and a density of 8 pcf or more.
Although fiber glass boards of the type described have functioned adequately as a form board, it would be desirable to have available a fiber glass board that does not deflect as much under load. Alternatively, if the amount of deflection of current boards is acceptable, such a board could provide a similar function but at a lower density. A major requirement in the manufacture of such a board is to be able to employ the same basic process utilized in manufacturing the current product in order to continue to derive the economic benefits of the process. Any modification of the process required to produce a board having improved strength and stiffness must therefore be compatible with the basic process.
It is known that loads applied to a fiber glass board are distributed within the board from fiber to fiber through resin bonded junctions. For optimum strength, therefore, each fiber should be long enough to intercept and be bonded to two or more adjacent fibers. At a particular binder level, assuming that the binder is strong enough to accept and transfer the fiber load, coarse fibers are preferable to fine fibers because they are straighter and can individually carry more load than fine fibers. Fine fibers, being present in greater quantity than coarse fibers at a given density, produce a greater number of fiber junctions which require a greater quantity of resin.
It would stand to reason that the process used to produce fiber glass form boards should provide predominantly long fibers. Unfortunately, the most economical processes available produce a mixture of long and short fibers which are not conducive to improved load distribution. For example, in the rotary fiberization process, by which molten glass is attenuated through small orifices in the side of a rapidly spinning metal cylinder to form fibers which are sprayed with binder as they fall to a moving collection conveyor, the reduction in rotational speed of the falling mass of fibers can cause long fibers to become entangled in clusters or bundles. The regions between bundles tend to have relatively low fiber content, resulting in areas of weakness in the board. One way of avoiding such areas is to add additional fiber to the collection conveyor to pack these regions. Another way is to produce very short fibers by use of a different process or by use of an air knife on the rotary process. Adequate fiber-to-fiber contact of short fibers, however, requires high fiber loading on the collection conveyor and relatively high binder content. In addition to being less economical than boards containing long fibers, boards comprised of short fibers tend to irritate the skin more and are less flexible.
The most desirable way of producing fiber glass boards having the strength and stiffness required for use as a form board would be to somehow modify existing processes without having to add fiber or produce boards comprised mainly of short fibers.